In a typical charge pump, a set of switches in a switching-network interconnects pump-capacitors to form different capacitor networks at different times. By transitioning between different capacitor networks, it is possible to transform an input voltage into an output voltage. A network of this type is often referred to as a “switched-capacitor network.”
The switches are typically implemented by field-effect transistors that can be made to transition between conducting and non-conducting states through the enhancement and depletion of an inversion layer between two similarly doped semiconducting regions. To cause this transition, it is usual to cause an electric field to be present in an oppositely-doped semiconducting region between the two similarly-doped semiconducting regions. The two similarly-doped regions are often called a “source” and “drain.” The oppositely-doped region is often called a “channel.”
To control the electric field, it is usual to place a conducting metal plate on an oxide layer that covers the channel region. This plate is often called a “gate terminal.” By depositing and withdrawing charge on this gate terminal, it is possible to control the electric field within the channel region. A circuit that is responsible for depositing and withdrawing this charge quickly and at particular times is a “driver.”
In general, when no voltage is applied, there will be no inversion layer to connect the source and the drain. As a result, a charge carrier faces a significant obstacle in trying to traverse the channel. This means that there is no significant conduction between the source and drain. When this happens, the transistor is conducting. In this state, the switch is said to be “open” or “off.”
To permit conduction, it is necessary to apply a voltage on the gate terminal. This voltage creates an electric field that drives away majority carriers, thus depopulating a layer near the gate terminal. Charge carriers from the source can then enter this depopulated layer and reach the drain from the source. Since this layer is now populated with what would be minority carriers from the gate's point of view, a population inversion, the layer is referred to herein as an “inversion layer.” When this happens, the transistor conducts. In this state, the switch is said to be “closed” or “on.”
In a transistor as described above, the switch is open by default. Such a transistor is referred to as “normally off.” In such cases, to close the switch, one needs to apply a voltage.
However, there also exist transistors in which this behavior is reversed. In these transistors, the switch is closed by default. Such a transistor is said to be “normally on.” To open the switch, one needs to apply a voltage. Such a transistor requires a different type of drive circuit.